1. Field of the Invention
The present invention relates to a hip joint prosthesis and more particularly to a femoral component of such a prosthesis which is coated with a material which reduces the bonding of the prosthesis to bone cement,
2. Description of the Prior Art
Many methods and devices have been developed to improve the fixation of hip joint prostheses including the femoral component thereof in the body so that the device implanted therein becomes as permanent as possible. Many orthopedic implants use a cement to anchor the stem portion of a femoral component in the femur. For example, United Kingdom Patent Specification No. 1,409,054 in the names of Robin S. M. Ling and Alan J. C. Lee discloses a hip joint prosthesis having a double-tapered stem which, among other advantages, enhances extrusion of cement caused by penetration of the stem during fixation. U.S. Pat. No. 3,793,650 discloses an intramedullary stem for a prosthetic bone joint device of this type also having a base with spring members intended to centralize the position of the stem in the canal or bore of the bone in order to insure a relatively uniform or, at least minimum thickness of cement between the wall of the bone and the stem. It has been found desirable to have a uniform mantle of at least two millimeters (2 mm) of cement between the stem and the bone.
The prior art has shown centralizers as a means for insuring that there will be at least a certain minimum thickness of cement between the stem of the prosthesis and the interior wall of the canal formed in the femur bone for receiving such stem, the likelihood of the stem protruding through the cement and contacting the interior of the femur bone itself is minimized. Thus, in those types of implants using cement, it is important to insure that the stem is completely encapsulated by the cement and does not protrude through to contact the bone.
One type of bone cement utilized to retain the stem of a femoral hip joint prosthesis in the canal of a bone comprises a mixture of polymethylmethacrylate (hereinafter PMMA) polymer and methyl methacrylate monomer and optionally including a styrene co-polymer of PMMA. This and other types of cement utilized for such purpose may be packaged in two separate components which are mixed into a paste which is placed in the canal of the femur immediately prior to insertion of the stem of the prosthesis. Such paste then sets to a relatively rigid material providing excellent adherence to the interior wall of the bone.
In both the cemented and non-cemented types of devices used heretofore, problems have arisen, particularly after a number of years of implantation. It has been found that the cement utilized to retain the stem of the device in the canal of the femur bone is subject to a phenomenon known as creep. Thus, while the bone cement appears to be rigid when set, it is subject to minute amounts of movement over time. The amount of creep encountered with such cement following implantation is exaggerated by virtue of the fact that the body temperature controls the temperature of the implanted cement and prosthesis. Thus, PMMA and other types of bone cement at body temperature are subject to a greater degree of creep than bone cement maintained at room temperature of, say, 72.degree. F. This may be readily observed by mounting a bar of PMMA so that its ends are supported and applying a fixed load at the center of the bar. Tests have shown that a bar so supported and subjected to a load of 5 pounds for eight hours at 98.6.degree. F. will deflect to an extent 3.5 times greater than an identical bar supported and loaded in an identical manner for eight hours at 72.degree. F.
Over a period of time, the phenomenon of creep may result in disruption of the micro-interlocking of the cement-implant interface. This may allow the prosthesis to loosen and cause unwanted movement. In addition, the femoral component could subside as the cement deformed over time.
In the early 1970's a polished femoral hip component was designed which was intended to subside as the bone cement deformed over time. This stem is shown in the afore-mentioned U.K. Patent 1,409,054 and sold under the trade name Exeter.RTM. hip by Howmedica Inc. With this design, the prosthesis is wedge-shaped and automatically relocks itself within the bone cement as subsidence occurs due to bone cement creep.
As discussed in U.S. Pat. No. 5,171,275, it is well known that polishing the Exeter.RTM. hip stem allows for less adhesion between the bone cement and the prosthesis to permit subsidence. It has now been unexpectedly found that coating a polished prosthesis with the coatings of the present invention further reduces the bond between the prosthesis and the bone cement.
Coatings such as diamond-like coatings are known and may be applied to metal by processes described in U.S. Pat. Nos. 4,382,100, 4,394,400 and 4,645,977, the teachings of which are incorporated herein by references. The diamond-like films produced by these methods on a metal substrate are known to reduce friction (see U.S. Pat. No. 4,525,417). The coating of metallic orthopedic implants has been broadly taught in EPO publication 0 302 717 A1 and Japanese Patent Application 59-82851 (1984) but these publications did not address the advantages of using such a coating on a polished hip stem of the type disclosed in U.K. 1,409,054 designed to subside as bone cement creeps.